In recent years, kinds of projectors are widely used in schools, homes and business occasions in order to amplify image signals provided by an image signal source and then display on a projection screen. For the purpose of reducing the power consumption and the size of devices, a solid-state light-emitting element is employed in the illumination system of current projector to replace the conventional high intensity discharge (HID) lamp.
Generally, the illumination system of the projector should emit three primary color lights, i.e. red light (R), green light (G) and blue light (B). As for the luminous efficiency of the solid-state light-emitting element, the luminous efficiency of the blue solid-state light-emitting element is higher than the luminous efficiency of the red solid-state light-emitting element and the luminous efficiency of the green solid-state light-emitting element. Since the blue solid-state light-emitting element has better luminous efficiency, the red light and the green light are produced by using a blue solid-state light-emitting element and a wavelength-transforming device to excite blue light as red light or green light (e.g. a phosphor wheel or a phosphor plate). That is, in replace of the red solid-state light-emitting element and the green solid-state light-emitting element, the uses of the blue solid-state light-emitting element and the device containing phosphor coating may directly emit the red light or the green light. Consequently, the luminous efficiency of the whole illumination system is enhanced.
For example, a single blue solid-state light-emitting element and a single phosphor wheel having a plurality of segments are used in current illumination system. FIG. 1A schematically illustrates the configuration of a conventional illumination system. FIG. 1B schematically illustrates the configuration of the phosphor wheel having a plurality of segments used in the illumination system as shown in FIG. 1A. As shown in FIG. 1A and FIG. 1B, a blue solid-state light-emitting element 11 of a conventional illumination system 1 emits blue light B to a phosphor wheel 12 including a first segment 121, a second segment 122 and a third segment 123. The first segment 121 is coated with a green phosphor agent in order to excite the incident blue light B as exit green light. The second segment 122 is coated with a red phosphor agent in order to excite the incident blue light B as exit red light. The third segment 123 is a transparent segment such that the blue light B is transmitted through the third segment 123. In other words, the blue light B emitted by the blue solid-state light-emitting element 11 is transmitted through the phosphor wheel 12 directly or is excited or transformed as green light or red light by the phosphor wheel 12, so that the illumination system emits the three primary color lights.
However, the red phosphor agent still has some drawbacks. For example, the cost of the red phosphor agent is higher and the performance in high working temperature is worse. Moreover, the red phosphor agent has a heavier weight than the green phosphor agent, so that the phosphor wheel coated with the red phosphor agent and the green phosphor agent has to be fixed of rotating balance. To overcome these drawbacks, another conventional illumination system equipped with a blue solid-state light-emitting element and a red solid-state light-emitting element is developed and projecting with the characteristic of dichroic mirrors. FIG. 2A schematically illustrates the configuration of another conventional illumination system. FIG. 2B schematically illustrates the configuration of the phosphor wheel as shown in FIG. 2A. Blue light B is emitted to a phosphor wheel 22 including a first segment 221 and a second segment 222 by a blue solid-state light-emitting element 21 of a conventional illumination system 2. The first segment 221 is coated with a green phosphor agent in order to excite the incident blue light B as exit green light G. The second segment 222 is a transparent segment such that the blue light B is transmitted through the second segment 222 and outputted. On the other hand, the illumination system 2 is also equipped with a dichroic mirror 24, which reflects red light and allows green light and blue light to be transmitted. Red light R emitted by a red solid-state light-emitting element 23 is reflected to the optical path by the dichroic mirror 24, and the blue light B and the green light G are passed through the dichroic mirror 24 and transmitted to the optical path. As a result, the blue light B, the green light G and the red light R are projected.
In some conventional illumination systems, the projecting structure and mechanism mentioned above can be used to operate with a common projector. Nevertheless, not only the overall fabricating cost, but also the fabricating time and difficulty of the illumination system will be increased due to the higher price, the higher weight and the worse performance in high working temperature of the red phosphor agent, which is coated on a segment of the phosphor wheel of the illumination system.
At the same time, when a driving current of a blue light laser of the above-mentioned illumination system is larger, the luminance and the illuminance of the red light will decay because of the characteristic of the red phosphor agent. Therefore, the luminance and the illuminance will be too low, such that the whole brightness cannot be integrated effectively, and further the sharpness and the contrast of color will be affected.
There is a need of providing an improved illumination system and a wavelength-transforming device thereof to obviate the drawbacks encountered from the prior art.